1. Field of the Invention
The present invention generally relates to a circuit for driving an ignition coil.
2. Description of Related Art
In a spark ignited internal combustion engine, ignition coils provide the voltage required for electrical current to jump across a spark plug gap. The spark ignites an air-fuel mixture in the engine cylinder causing combustion. A switch, also referred to as a coil driver, is used on the primary side of the ignition coil to control the charge and discharge cycles of the ignition coil.
A typical ignition system is illustrated in FIG. 1. The system includes an ignition coil 210 having a primary side 216 and a secondary side 218. The positive terminals of the primary side 216 and secondary side 218 of the ignition coil 210 are connected to a power source 212. The negative terminal of the primary side is connected to a switching transistor 214. The switching transistor 214 is connected between the ignition coil 210 and an electrical ground 220. The negative terminal of the secondary side 218 is connected to a spark plug 222. The spark plug 222 is connected between the ignition coil 210 and an electrical ground 220.
FIG. 2 illustrates the voltage and current profiles at various points within the prior art system. The profile of the control signal provided to the switching transistor 214 is identified by reference numeral 224, while the current flowing through the primary side 216 of the ignition coil 210 is denoted by reference numeral 226. In addition, a profile of the primary coil voltage signal, as seen on the collector of transistor 214, is denoted by reference numeral 228. In a typical charge and discharge cycle the switching transistor 214 is turned on, charging the ignition coil 210 for a specified dwell period or to a specified charge current; and then the switching transistor 214 is turned off, allowing the secondary side 218 of the ignition coil 210 to discharge stored energy across the spark plug gap.
One problem is that the sharp turn-on during the charging cycle causes an oscillation on the secondary side 218 of the ignition coil 210. FIG. 3 illustrates the dwell command signal 224, the dwell current 226, low-side voltage 228, and the undesirable secondary voltage oscillation 230. The switching transistor 214 starts out in the off-state with the negative terminal of the primary side 216 equal to the battery voltage. After the switching transistor 214 is turned on, the transistor quickly transits through its linear range into the saturated on-state with very large voltage change across the primary side 216 of the ignition coil 210. The resulting secondary voltage 230 during turn-on is a large oscillation magnitude that decays in time. If the oscillation magnitude exceeds a tolerable level, an unintended spark event can occur across the spark plug gap, resulting in premature combustion. One way to control the magnitude of the secondary oscillations is adding constraints in design of the ignition coil 210. These constraints, however, result in poor coil performance.
In view of the above, it is apparent that there exists a need for an improved circuit for driving an ignition coil.